Ballistics

Ballistics is the area of forensic science that deals with the study of firearms, ammunition, and bullet trajectory (flight path). Firearms include handguns (revolvers or semiautomatics), rifles, and shotguns. Much of ballistics involves what happens when a weapon is fired. Pulling the trigger of a firearm causes its firing pin to strike the primer that is located at the base of the cartridge. The ignition of the primer detonates gunpowder within the cartridge, resulting in its conversion into a gas. The rapid increase in pressure within the cartridge causes the bullet to simultaneously expand and be propelled through the barrel of the gun.

The barrel of a firearm is manufactured with helical grooves carved into its inner surface. As a result, most of the barrel contains lands and grooves. Lands refer to the high points of the inner surface between adjacent grooves. The spiraled grooves cause the bullet to rotate laterally as it is propelled through the barrel. The imparted spin of the bullet helps provide stability and prevent wobbling or tumbling as it traverses through the air with minimal loss of velocity and without changing direction along its trajectory. Twist refers to the length of the barrel per revolution. Twist can be left-or right-handed, and the barrel may be tooled with different numbers of grooves. The caliber of a gun and its ammunition refers to the internal diameter of the barrel. It is usually expressed in fractions of an inch(e.g., 0.38-caliber) or in millimeters (e.g., 9 mm). At times, the examiner may not be able to characterize a bullet that has become severely deformed as a result of having struck a hard object. Instead, the bullet can be weighed. The number of grains can be used to determine its caliber (1.0 grain = 1/7000 of a pound). All of the above factors are called class characteristics and are used to describe the barrel and bullets fired from a firearm. A barrel with six right-handed grooves would create a bullet with six striations with a right-handed twist.

It should be no surprise that a bullet in flight does not travel in a straight line. The bullet must obey the laws of physics. In addition to the forces propelling the bullet forward, the force of gravity simultaneously pulls the bullet downward. The bullet's velocity (measured in feet per second, or feet/sec) is affected by a number of factors, including the frictional forces caused by the air. Some bullets, because of their shape and design, are more aerodynamic and will have less drag than a blunt-nosed or irregularly shaped bullet. A bullet's velocity depends in part on the amount of drag present. A bullet that has a larger mass and is more aerodynamically designed will travel faster than one with smaller mass and/or an irregular shape. Wind can also have an effect on a bullet's trajectory by causing the [Page 22]bullet to drift. The ballistic coefficient is a number that reflects a bullet's length, diameter, and shape and indicates its ability to overcome air resistance. A ballistic coefficient of 1.0 indicates that this bullet will retain its velocity and energy, and exhibit a relatively flat trajectory, which is an ideal characteristic when the target is at a relatively far distance. Most bullets have coefficients between 0.7 and 1.0. The shape of the bullet can affect not only its maximum speed, but also its penetrating power and degree of destruction of the target.

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